Wednesday, December 4, 2013
Sapling Problem #28
The problem i chose to workout was problem number 28 on Sapling. This is a synthesis problem in which you are asked what reactants and in what steps you could form (E) 1,2-dibromo-1-butene from acetylene.
If you follow my arrows i will explain each step. The first thing i notice when looking at the starting material and the final product are the number of Carbons in each. We are starting with an acetylene with 2 carbons and ending with product with 4 carbons. With this known i used the 1.NaNH2 and 2.Alkyl halide reactants in order to add carbons. Since we need two additional carbons i used an ethyl bromide. The next thing that needs to happen is the anti addition of the 2 bromines. This can be done simply by adding 1 equivalence of Br2. This ultimately gets us to our (E)1,2-dibromo-1-butene.
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I liked how in your diagram you separated out the intermediate products after adding NaNH2, which ultimately deprotonates the alkyne. For alkyl addition steps I always just draw an arrow with a '1.NaNH2' on the top and a '2. alkyl halide' on the bottom, so in the beginning when we first started doing these synthesis problems I had to remind myself that the numbers were necessary because it wasn't a concerted reaction - one had to occur before the other. So the way you drew it out makes it really clear to see what is happening in those first couple of steps.
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